Insulated overhead door for trucks and the like

ABSTRACT

An overhead door assembly including a pair of spaced panels having insulating material located between the inside surfaces and holding the panels together. The insulating material is resilient to allow differential movement of the panels at least longitudinally of each other and preferably also laterally toward each other as the door is bent while moving between closed and open positions.

United States Patent [191 Huprich [451 Apr. 3, 1973 [s41 INSULATED OVERHEAD Door: FOR

TRUCKS AND THE LIKE [76] Inventor: Carl A. Huprlch, 630 N. Buys Road,

Muskegon, Mich. 49445 [22] Filed: Dec. 16, 1971 [21] Appl. No.: 208,737

[52] us. Cl. ..160/368 n 51] Int. Cl. ..A47h 3/00 58 Field of Search ..160/368 R,179;49/34,197

[56] References Cited UNITED STATES PATENTS 1,553,735 I 9/1925 Wolft ..49/34 2,392,715 1/1946 Zitrin ..l60/179 2,258,971 10/1941 Carlson l ..l60/363 R 2,258,972 10/1941 Carlson ...l60/368 R 2,589,938 3/1952 l-lanrahan ...l60/368 R 3,169,574 2/1965 Behlen ..160/368 R Primary Examiner- Peter M. Caun Attorney-Price, Heneveld, l-luizenga & Cooper [57] ABSTRACT An overhead door assembly including a pair of spaced panels having insulating material located between the inside surfaces and holding the panels together. The insulating material is resilient to allow differential movement of the panels at least longitudinally of each other andpreferably also laterally toward each other as the door is bent while moving between closed and open positions.

19 Claims, 9 Drawing Figures PATENTEDAPRS I875 SHEET 1 [1F 2 PATENTED APR 3 I975 SHEET 2 [IF 2 INSULATED OVERHEAD DOOR FOR TRUCKS AND THE LIKE BACKGROUND OF THE INVENTION Various types of closure members or doors have been devised for closing the large opening in refrigerated trucks or the like. Ideally, the door will be relatively light in weight, yet be sufficiently rigid to withstand the severe environment to which it is exposed. In addition, the door must be insulated to maintain an artificial environment within the truck body while at the same time protecting the materials contained therein from the outside environment. Overhead doors for such purposes, are generally in the form of a plurality of individual rigid panel assemblies extending across the opening and hinged together such that they may be moved vertically upwardly and into a horizontal position.

One overhead door structure is illustrated in U.S. Pat. No. 3,017,218 wherein, an outer panel of flexible sheet metal is spaced from an inner panel formed of a plurality of inner strips having a longitudinal edge hingedly connected to an adjacent strip. Spacer members are provided between the outer panel and the inner strips and heat insulating material is positioned in the space between the outer panel and inner strips.

Another overhead door construction is illustrated in U.S. Pat. No. 2,258,971 wherein a pair of panels rigidly connected at their lower portion are adapted to slide upwardly and horizontally along a pair of channel-like members. Each of the panels are individually carried in a pair of opposed channels which,at the upper portion LII thereof, curve inwardly to join an upper horizontal channel and are wider than the thickness of the door to prevent binding of the panels as they move upwardly.

SUMMARY OF THE INVENTION The present invention provides an insulated overhead door assembly wherein a pair of opposed parallel channel track members having their open sides facing each other are positioned at either side of the door opening. The channel members extend upwardly and at their upper ends are curved to extend horizontally so that the door may be moved upwardly and horizontally within, for example, a truck body. The door itself in- I cludes spaced panels of a flexible material, preferably with a resilient insulating material positioned between them. The insulating material interconnects the two panels and yet allows movement betweenthe panels such that they are movable toward each other and longitudinally of each other when the door is bent while passing through the curved portions of the track. The panels are interconnected so that they shift longitudinally with respect to each other to compensate for one of the panels traversing a greater radius of curvature than the other panel. Opening of the door may be facilitated by spring means attached to, for example, the body of a vehicle and to the top end of the door. Alternately, a torsion bar assembly having cables attached to the bottom edge of the door may be provided. In an alternate embodiment of the door, resilient sealing'means may be provided at the edges of the panels to form an air-tight seal. The space between the panels may be filled with air to provide a thermal barrier rather than a solid insulating means. The panels themselves may be constructed of sheet metal, fiber glass, plastic or other non-elastic but flexible material.

In another embodiment of the invention, a plurality of horizontally positioned stiffening and tension members may be embedded within the insulating material between the panels. The members extend outwardly where rollers affixed at the ends thereof ride in C- shaped track members fixed between the parallel channel track members in which the panels are slidably mounted.

To seal the door when closed, the lower vertical portions of the channel members may be slightly narrower than the thickness of the door such that as the door is pulled downwardly, the longitudinal movement of the panels with respect to each other will allow the door to close, while at the same time its own weight will tend to fill the space between the channels, thereby creating a weather-tight seal between the panels and the channels.

Accordingly, it is a primary objectof the present invention to provide an insulated door structure for use in refrigerated truck bodies or the like.

It is another object of the invention to provide an insulated door structure having a pair of relatively movable panelmembers which are movable longitudinally of a each other so that the panel can be accommodated within the curved portions of the track members of the door structure as it is moved from a vertical to a horizontal position.

It is also an object of this invention to providesuch structure in which the panel members are not only movable longitudinally but also laterally toward and away from each other.

It is yet another object of the present invention to provide a door structure-uniquely constructed and arranged within its track members such that a weathertight seal is provided when the door is closed.

Other objects and the many advantages of the present invention will be readily appreciated by those skilled in the art as the construction and operation of the door, becomes better understood by reference to the following detailed description and the accompanying drawings illustrating a preferred embodiment of the invention.

BRIEFDESCRIPTION OF THE DRAWINGS The invention will now be described in greater detail with reference to the accompanying drawings in which:

FIG. 1 is a fragmentary, vertical sectional view of a door structure in accordance with the invention;

FIG. 2 is a cross-sectional view taken along the plane Il-Il of FIG. 1; a

FIG. 3 is a cross-sectional view illustrating a first embodiment of the present invention;

FIG. 4 is a view similar to FIG. 2 illustrating a second embodiment of the door'construction of the present in vention;

FIG. 5 is a cross-sectional view similar to FIGS. 3 and 4 illustrating a third embodiment of the present invention;

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, a body on a refrigerated truck or like vehicle is illustrated and generally designated by the numeral 10. The body is provided with an opening 12 in the rear thereof and is adapted to be closed by a door 14. The door 14 is mounted so that it may be moved between a vertical or closed position and a horizontal or open position, in a pair of opposing parallel U-shaped channel track members 16 positioned on either side of the opening 12. The track members include a vertical section 17 within the door opening and a horizontal section 20 in the upper part of the vehicle body. The vertical and horizontal sections of the track members are connected by a curved track section 22.

Referring briefly to FIG. 2, the track members 16 each include an inner flange 24 and an outer flange 26 connected by a base portion 18. The track members are secured in any convenient manner as by screws 27 in the framework of the door opening and to overhead structural members of the body.

Referring to FIG. 3, the construction of the door itself will be described in detail. Basically, the door includes an outer panel 28 and an inner panel 30, each of which are constructed of generally nonelastic flexible materials of substantially the same size. By nonelastic is meant that the material will not stretch longitudinally and then recover or spring back to its original size, and it should be understood that nonelastic material as used herein will bend and may or may not have a tendency to recover or spring back to its original bent or nonbent condition. The panels may be, formed of sheet metal, fiber glass or other suitable nonelastic, flexible material. In the vertical position shown, the panels are arranged parallel to each other. A plurality of thin, elongated stiffening members 32 are positioned across the width for transverse reinforcement of the panels. The stiffening members 32 may be made of wood, thin metal bars, or other suitable material and connected by adhesives or other convenient means to the inner facing portions of the panels 28 and 30. The stiffening members may extend across the entire width of the door or they may be arranged into a plurality of short transversely extending portions.

The open space between the walls 28 and is filled with a suitable flexible insulating material 29. In a preferred embodiment of the invention, a flexible polyurethane material is foamed into place between the panels and allowed to cure. Actually, any one of the many elastomeric materials is suitable provided that it has good insulating properties and is capable of adhering to the panels to hold them in a spaced-apart position. The insulating material serves two purposes: It prevents the entrance of heat or cold into the body of the truck when the door is closed and, in addition, it holds the two panels together, spacing them apart and yet allowing differential movement between them. When assembled, the panels 28 and 30 are movable toward each other as the insulating material 29 is compressed or deformed as the door passes through the .curved track section 22. In addition, the doors are movable longitudinally of each other as will be more fully described hereinafter.

The insulating material is foamed between the panels.

utilizing conventional insulation foaming techniques well known to those skilled in the art. Basically, urethane foams are provided by polymerizing a diisocyanate and a polyol in the presence of a blowing agent. The panels may be held in a fixture so that the sides are completely closed and the material foamed between the panels. When the insulating material has I cured, the formis removed. The insulating material remains flexible yet forms an integral structure with the panels 28 and 30.

In an alternate embodiment of the invention as shown in FIG. 4, an inert gas or air is used as the insulating material by providing a dead air space between the panels. In this embodiment, it is necessary to completely enclose the area between the panels. The construction of the panels 28 and 30 and the stiffening ribs 32 is substantially identical to that shown in FIG. 2. To seal the door, however, it is necessary toprovide side and end closures for the panels. A thin flexible sealing membrane 34 is provided at the top, bottom, and about the edges of the panels. The sealing membrane 34 may be heat sealed or secured by adhesives to the panels 28 and ,30. A filler tube 36 is provided on the inner panel 30 where air or other inert gas is introduced between the panels. After filling the space between the panels with a suitable gas, the tube 36 is pinched off and sealed in a conventional manner.

Referring again to FIG. 2, the inner facing portions 24 and 26 of the channel member 16 may be coated with a lubricous material 36 to reduce friction between the channel members and the surfaces of the door panels. An ideal material for this purpose is a film of tetrafluoroethylene fluorocarbon polymer (TFE). This material is commercially available under the tradename Teflon, a trademark of the E. I. duPont de Nemours & Co.

Referring again to FIG. 1, the door assembly may include a tension spring 38 located at the outermost end of the horizontal portion 20 of the track members. The spring is fixed at one end (not shown) to the body of the vehicle 10 while the other end is connected to the upper portion of the door 14. The spring is operatively connected to the door by means of a hook-like member 40 extending from a curved channel-like member 42 secured between the panels at the upper surface of the door. The curved channel 42 is secured to the panels by rivets or the like. The hook member 40 passes through an elongated slot 44 on the radius of the curved channel (FIG. 3) where it is secured by a nut and washer 16. The elongated slot allows relative rotation of the channel 42 about the hook 40 when the panels move longitudinally with respect to each other as will be more fully described hereinafter. The hook member may alternately be embedded in the insulating material or fixed to a portion of the internal structure such as, for

example, one of the stiffening ribs 30 within the door neatthe upper end.

A torsion spring assembly 48 may also be provided to assist in the upward movement of the door from the vertical to the horizontal positions. Torsion spring devices are well known to those'skilled in the art and a detailed explanation is not believed necessary herein. Basically, however, the torsion assembly 48 includes a spring-loaded drum 50 rotatably mounted on a fixed shaft 52 over which a cable 54 is wound. An extending end of the cable 56 is connected to the lower edge surface of the door. As the door is raised from the vertical position toward the horizontal, the spring rotates the drum winding the cable on the drum to assist in movement of the door toward the horizontal position. Depending upon the size of the door and the particular structure of the body that the door assembly is mounted, either or both the tension spring 38 or the torsion spring assembly 48 may be utilized in connection with the door to assist in the opening and closing thereof.

An important feature of this invention is depicted in FIG. 1 wherein the door is shown being opened from a vertical to a horizontal position. As the door. passes through the curved section 22 of the channel members, the inner panel 30 travels through a first radius of curvature R while the outer panel 28 passes through a second radius of curvature R. Due to the unique construction of the door panels, the panel sections 28 and 30 are allowed to move toward each other while they pass through the curved section 22. The panels are not distorted or deformed in any way as they move toward each other and shift longitudinally of each other since the insulating material 29 is flexible. Referring to the upper or leading edge of the door where connection is made with the tension spring 38, the inner panel 30 has moved a slightly greater distance within the upper channel track portion than has the outer panel 28. Similarly, as viewed in the right-hand portion of the figure in the vertical section 17, the outer panel member 28 has not moved as far upwardly as the inner panel 30. The panel members have shifted longitudinally with respect to each other to compensate for and to accommodate the differing radii of curvature of the panels in the curved portion 22 of the track members.

The flanges 24 and 26 on the vertical section 17 of the channel track members are spaced closer to each other at the lower portion of the vertical section and taper outwardly until they meet with the curved section 22. This tapering provides an extremely effective and air-tight seal for the door when it is closed. As the door 14 is pulled downwardly by grasping the conventional handle assembly 60, a wedging effect takes place as the panels move within the lower portion of the vertical section. Slight movement of the panels 28 and 30 with respect to each other occurs tightly wedging the door assembly between the track members to thereby provide a weather-tight seal.

In the third embodiment of the invention illustrated in FIGS. 5 through 9, similar reference numerals bearing the suffix letter a" are utilized to designate similar parts.

Basically, the third embodiment includes a pair of adjacent, non-elastic flexible panels 28a and 30a filled with a suitable flexible insulating material 29a. A plurality of horizontal stiffening members 70 are embedded within the insulating material across the width of the panels. As shown in FIG. 6, each vertical stiffening member 70 has end portions 82 extending from the panel edges where roller members 72 are attached. A plurality of vertically positioned stiffening members 74 may also be positioned between the panels and interwoven as shown between the horizontal members 70 to impart additional rigidity and strength to the structure if desired.

Referring to FIGS. 7 and 8, the roller members 72 are preferably anti-friction bearings having an inner race 76 and an outer race 78 separated by a plurality of rolling elements 80. The stiffening members 70 are formed of material preferably of about 20 gauge thickness. The ends 82 extending from between the I panels and insulating material are rolled over and inserted through the opening in the inner race 76 of the bearing. The end 82 is held in place within the inner race by means of a conventional expandable rivet 84 having a head 86 and a shank portion 88 engaging and securing a wedge portion 90 to fix the end 82 of the member'70 within the opening in the inner race of the bearing. v

In this embodiment of the invention, C-shaped track members 92 are positioned in the U-shaped track member 16a between the inner and outer flanges 24a and 26a and held in position by screws 27a fixed in the framework of the door opening and to the overhead structural members of the body. The C-shaped track 92 extends along the entire length of the U-shaped track 16a on both sides of the opening 12 in the vertical sections 17 and upwardly through the curved portion 22 into the horizontal overhead section 20 (FIG. 1

Preferably, the stiffening members are placed in slight tension when positioned between the track members 92 to rigidly hold the assembly in position. This is accomplished by the abutment of the inner facing portions of the outer race of the bearing abutting against a pair of inwardly formed flanges 94 on the track member 92. The opening between the. flanges allows passage of the ends 82 of the members 70. t

y In addition to making the panel assembly more rigid than that shown in the previously described embodiment, this embodiment also serves as additional function of creating a safety barrier within the closure member such that if freight loaded within the vehicle should break loose, the structure formed by the stiffening members 70 and 74 would absorb the impact and prevent the panels from being pushed out of the opening. This feature alone when installed in a conventional truck body allows the elimination of the wire mesh safety gate commonly used in such vehicles. While the tension members impart additional strength and rigidity to the closure structure, they do not obstruct or hinder its operation in any way as the door moves between open and closed positions through the curved portion 22 of the channel members. It will'be readily recognized that in this'embodiment, the members 70 and 74 may be positioned virtually anywhere between the panels and if desired, may be laminated to one of the panel members itself and yet operation of the panels will remain simple and efficient. Depending, of course, upon the positioning of the members 70 within the panel, the track members 92 will be shifted within the track 16a. As in the previous embodiments, the inner facing portions of 24a and 26a of the channel members 16a may be coated with a lubricous material 36a to reduce friction.

It will become readily obvious to those skilled in the art that the door of this invention does not involve the use of hinges, multiple panels or like devices in its provision of an overhead sliding door. The door operates extremely simply in its movement from open to closed position. Upward movement may be assisted by tension springs or a torsion assembly. The door presents an unbroken exterior surface which may be provided with advertising material or the like as desired. The inner surface of the door also presents a clear, unbroken surface to the inner portions of the refrigerated compartment. This is particularly desirable from a cleanliness standpoint as, for example, when handling bulk food products such as meat and the like. Prior art door closure members having a plurality of panels hingedly connected to each other tend to attract dirt, moisture and the like requiring extreme care and effort to clean. Obviously, the clear, unbroken surface presented by the inner facing door panel of the present invention is easy to clean and maintain. The door is relatively light in weight, is simple to construct, is easy to maintain and has no moving parts to become worn, requiring replacement.

As will be readily appreciated by those skilled in the art, many of the modifications and variations of the present invention are possible in light of the above teachings. Accordingly, modifications embodying the principles of this invention are to be considered as included in the appended claims unless these claims, by their language, expressly state otherwise.

The embodiments of the invention in which anexclusive property or privilege is claimed are defined as follows: I

1. An insulated overhead door assembly for a door opening comprising:

a pair of opposed parallel channel track members having their open sides facing one another, each of said track, members having vertical portions, horizontal portions and curved portions connecting said horizontal and said vertical portions; and

a door having spaced panels, the side edges of said door being slidably received respectively within the open sides of said track members so that said door is slidable in said track members and said panels being interconnected by means permitting differential movement thereof longitudinally of each other so that as said door is bent while passing through said curved portions of said track members, said panels are shifted longitudinally of each other to compensate for one of said panels traversing a greater radius of curvature than the other of said panels as said door is moved through said curved portions of said track members.

2. An insulated overhead door assembly for a door opening comprising:

a pair of opposed parallel channel track members having their open sides facing one another, each of said track members having vertical portions,

horizontal portions, and curved portions connecting said horizontal and said vertical portions; and door having spaced panels of nonclastic flexible material of substantially the same size, and a resilient heat and cold insulating material therebetween; the side edges of said door being slidably received respectively within the open sides of said track members so that said door is slidable in said track members and said panels being interconnected by means permitting differential movement thereof toward each other and longitudinally of each other so that as said door is bent while passing through said curved portions of said track members, said panels are moved toward each other at the curved portions of said track members and said panels are shifted longitudinally of each other to compensate for one of said panels traversing a greater radius of curvature than the other of said panels as said door is moved through said curved portions of said track members.

3. The door assembly according to claim 2 wherein each of said panels has a plurality of spaced side-to-side stiffening member secured thereto.

4. The door assembly according to claim 2 wherein said means interconnecting said panels is said insulating material which is adhered to both of the inside surfaces of said panels.

5. The door assembly according to claim 2 which further comprises biasing means connected to said door to facilitates sliding said door.

6. The door assembly according to claim 5 wherein said biasing means comprises a torsion bar assembly.

7. Thedoor assembly according to claim 5 which further comprises spring means affixed to the body of said vehicle and the upper end of said door to facilitate opening of said door.

8. The door assembly according to claim 2 which further comprises resilient sealing means between and at the peripheries of said panels to form an airtight chamber therebetween and wherein said insulating material is air.

9. The door assembly according to claim 2 wherein said panels are sheet metal.

10. The door assembly according to claim 2 wherein said panels are fiberglass.

11. The door assembly according to claim 2 wherein said insulating'material is a polyurethane foam.

12. The door assembly according to claim 2 wherein the open sides of said vertical portions of said track members are tapered from a width at the top of approximately the normal thickness of said door to a width at the bottom less than the normal thickness of said door so that as said door is closed, said door is wedged into said tapered tracks to form an air and water-tight seal between said tracks and said door.

13. The door assembly according to claim 2 wherein said open sides of said track member are lined with a lubricous material to facilitate sliding of said door therein. I

14. The door assembly according to claim 2 which further comprises a plurality of spaced stifiening members between said panels, said stiffening members extending horizontally within said panels having end portions extending from the sides of said panels; roller means fixed to the ends of said stiffening means; and a second pair of opposed channel track members positioned within said first-mentioned track members and adapted to receive said roller membersv 15. The apparatus according to claim 14 and further including a plurality of spaced-apart stiffening members extending vertically between said panels cooperable with said horizontal stiffening members to impart structural rigidity to said door.

16. The apparatus according to claim 14 wherein said horizontal stiffening members are fixed between said panels by said insulating material.

17. The apparatus as defined in claim 14 wherein said roller members include an inner race and an outer race, said inner race having a central opening therein; said extending ends of said stiffening members extending through said central opening in said inner race; and

means securing said end of said stiffening members in said inner race.

18. The apparatus as defined in claim 17 wherein said extended end ofsaid stiffening members positioned in said inner race is curved for reception in said central opening in said inner race and said fastening means is a rivet having a portion thereof expanded within said inner race central opening.

19. The apparatus as defined in claim 17 wherein said second pair of opposed channel track members each include a pair of inwardly directed flange mem bers adapted for abutment against a side of said bearing members whereby said stiffening members are held in tension between said track members between said panels. 

1. An insulated overhead door assembly for a door opening comprising: a pair of opposed parallel channel track members having their open sides facing one another, each of said track members having vertical portions, horizontal portions and curved portions connecting said horizontal and said vertical portions; and a door having spaced panels, the side edges of said door being slidably received respectively within the open sides of said track members so that said door is slidable in said track members and said panels being interconnected by means permitting differential movement thereof longitudinally of each other so that as said door is bent while passing through said curved portions of said track members, said panels are shifted longitudinally of each other to compensate for one of said panels traversing a greater radius of curvature than the other of said panels as said door is moved through said curved portions of said track members.
 2. An insulated overhead door assembly for a door opening comprising: a pair of opposed parallel channel track members having their open sides facing one another, each of said track members having vertical portions, hoRizontal portions, and curved portions connecting said horizontal and said vertical portions; and a door having spaced panels of nonelastic flexible material of substantially the same size, and a resilient heat and cold insulating material therebetween; the side edges of said door being slidably received respectively within the open sides of said track members so that said door is slidable in said track members and said panels being interconnected by means permitting differential movement thereof toward each other and longitudinally of each other so that as said door is bent while passing through said curved portions of said track members, said panels are moved toward each other at the curved portions of said track members and said panels are shifted longitudinally of each other to compensate for one of said panels traversing a greater radius of curvature than the other of said panels as said door is moved through said curved portions of said track members.
 3. The door assembly according to claim 2 wherein each of said panels has a plurality of spaced side-to-side stiffening member secured thereto.
 4. The door assembly according to claim 2 wherein said means interconnecting said panels is said insulating material which is adhered to both of the inside surfaces of said panels.
 5. The door assembly according to claim 2 which further comprises biasing means connected to said door to facilitates sliding said door.
 6. The door assembly according to claim 5 wherein said biasing means comprises a torsion bar assembly.
 7. The door assembly according to claim 5 which further comprises spring means affixed to the body of said vehicle and the upper end of said door to facilitate opening of said door.
 8. The door assembly according to claim 2 which further comprises resilient sealing means between and at the peripheries of said panels to form an airtight chamber therebetween and wherein said insulating material is air.
 9. The door assembly according to claim 2 wherein said panels are sheet metal.
 10. The door assembly according to claim 2 wherein said panels are fiberglass.
 11. The door assembly according to claim 2 wherein said insulating material is a polyurethane foam.
 12. The door assembly according to claim 2 wherein the open sides of said vertical portions of said track members are tapered from a width at the top of approximately the normal thickness of said door to a width at the bottom less than the normal thickness of said door so that as said door is closed, said door is wedged into said tapered tracks to form an air and water-tight seal between said tracks and said door.
 13. The door assembly according to claim 2 wherein said open sides of said track member are lined with a lubricous material to facilitate sliding of said door therein.
 14. The door assembly according to claim 2 which further comprises a plurality of spaced stiffening members between said panels, said stiffening members extending horizontally within said panels having end portions extending from the sides of said panels; roller means fixed to the ends of said stiffening means; and a second pair of opposed channel track members positioned within said first-mentioned track members and adapted to receive said roller members.
 15. The apparatus according to claim 14 and further including a plurality of spaced-apart stiffening members extending vertically between said panels cooperable with said horizontal stiffening members to impart structural rigidity to said door.
 16. The apparatus according to claim 14 wherein said horizontal stiffening members are fixed between said panels by said insulating material.
 17. The apparatus as defined in claim 14 wherein said roller members include an inner race and an outer race, said inner race having a central opening therein; said extending ends of said stiffening members extending through said central opening in said inner race; and means securing said end of said stiffening members in said inner race.
 18. The apparatus as dEfined in claim 17 wherein said extended end of said stiffening members positioned in said inner race is curved for reception in said central opening in said inner race and said fastening means is a rivet having a portion thereof expanded within said inner race central opening.
 19. The apparatus as defined in claim 17 wherein said second pair of opposed channel track members each include a pair of inwardly directed flange members adapted for abutment against a side of said bearing members whereby said stiffening members are held in tension between said track members between said panels. 